[Turbofan] Add USE for local used only by DCHECK.

src/compiler/register-allocator.cc

Index: src/compiler/register-allocator.cc
diff --git a/src/compiler/register-allocator.cc b/src/compiler/register-allocator.cc
index 977953509a02b49b71ea65a89ad4c4109e40b671..8661b192d51738553a89a589eca6abc40c6d8efd 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -1936,6 +1936,7 @@ TopLevelLiveRange* LiveRangeBuilder::FixedFPLiveRangeFor(
   }
 
   DCHECK(index < num_regs);
+  USE(num_regs);
   TopLevelLiveRange* result = (*live_ranges)[index];
   if (result == nullptr) {
     result = data()->NewLiveRange(FixedFPLiveRangeID(index, rep), rep);